The Warehouse Blenheim manager, Ian Daubney said there glass had smashed all over the floor and a lot of stock had been damaged. The store was evacuated and closed briefly while staff cleaned up the mess.

"It was huge,'' he said. "It shook our staff something wicked.''

The store had been re-opened so people could stock up on emergency items such as batteries and water, he said

The crew on a 10m yacht coming back into Waiakawa thought they had run aground at the entrance to the marina and discovered later that is was at the time of the earthquake. They said it was remarkable to have felt the quake on the water.

A Blenheim woman said she was driving when the quake hit, it felt like the car had a flat tyre and it felt like she was about to hit the curb.

Central New Zealand has been shaken throughout the day with a swarm of quakes including this evening's 6.8 event.

The majority of the quakes have been centred in the same area off Seddon, Marlborough, that was hit on Friday by a 5.7. This morning's first quake - 4.2 - struck at 7.04, was at a 10km depth and centred 35km east of Seddon.

It was followed 13 minutes later by the 5.8 shake that was centred 30km east of Seddon at a depth of 19km and then at 7.20 by a 4.3km shake at a depth of 14km and 25km east of Seddon, while a fourth - 4.3 - rattled the region from a depth of 22km at 7.30.

There have been over 30 earthquakes in the same area in total so far this morning with the latest bigger ones being 4.5 magnitude at 9.28am, 5.1 at 10.55am and 4.6 at 12.21pm.

"It's quite common to get a shock reaching about four (on the richter scale) before a larger one, like we had before the 5.8 this morning.

"It's also normal to get a series of threes and fours afterwards and about every two minutes or so is common.''

GeoNet's website also crashed this morning due to a high demand from people wanting to see where earthquakes have struck.

The run of earthquakes are a reminder that Wellington is a seismic active area and everyone should always be prepared, she said.

"These quakes this morning are very close to the subduction interface, which is the boundary between the Pacific and Australian plate."

The aftershocks this morning have ranged from 2.5 to 4.1 - all centred in the same region. While Geonet initially reported quakes in other areas in both islands, it appears they were erroneous recordings sparked by the other quakes.